It is often desired to provide a single antenna having directional performance in multiple frequency bands. Many radio services have assigned frequencies in bands of frequencies scattered through the usable radio spectrum. One example is the amateur radio service that has frequency assignments in various high-frequency bands including bands centered at or near 10, 12, 15, 17, 20, 30, 40, 80 and 160 meters. Directional and rotatable parasitic array antennas are widely used by radio amateurs in the 10 through 40 meters bands. Although separate directional and rotatable antennas for each band ("monoband beams") are used by many radio amateurs, it is more common to use multiband parasitic arrays, particularly for the 10, 15 and 20 meter bands (a so-called "triband beam," "triband yagi" or, simply "tribander" in the case of a Yagi-Uda type antenna or a "triband quad," in the case of a quad type antenna) (both the yagi antenna and the quad antenna are endfire multielement array antennas, the yagi employing half-wave dipole elements and the quad employing full-wave loop elements, typically in a square or diamond shape).
In order to minimize space, weight and cost, triband beams and quads typically employ a single support boom with yagi or quad elements, respectively, spaced along the boom. In the case of a triband yagi, multiband operation may be achieved by interlacing dedicated yagi elements for each band or by employing yagi elements having "traps" so that one element operates on three bands.
"Traps" are parallel-resonant circuits located at two symmetrical points with respect to the midpoint of a yagi dipole element. Traps decouple a portion of the element automatically as the antenna operation is changed from band to band. The high impedance of the parallel resonant circuit near its resonant frequency isolates or decouples unwanted portions of the antenna element. Thus, for a 10/15/20 meter triband beam, a first set of traps, the inboard traps, resonate at 10 meters and are located so that only the central portion of dipole element, resonating at 10 meters is active. The second set of traps, the outboard traps, resonate at 15 meters, and are located so that the central portion of the dipole element in combination with the shortening effect of the 10 meter traps and a further length of element between the 10 and 15 meter traps are active and resonate at 15 meters. The remaining portion of the element has a length such that the overall combination resonates at 20 meters. A common configuration is a so-called "three element trapped triband beam" in which each of three elements, a reflector, a driven element and a director each employ traps in order to provide three element yagi operation on 10, 15 and 20 meters.
While providing reasonable gain and directivity for their size, weight and cost, three element trapped triband beams are subject to inherent shortcomings, including, for example, the inability to optimize performance for all three bands (the same element spacing is necessarily required for all three bands) and losses in the traps themselves.
It is also know to use a combination of one or more trapped elements with interlaced, untrapped elements in an attempt to overcome some of the shortcomings of three element triband beams.
Another approach to three band operation, briefly mentioned above, is to eliminate all traps and interlace only non-trapped elements. While this approach has the benefit of eliminating trap loss, other problems arise from the interaction of the larger number of elements resulting from cross coupling. A three element triband beam configured with non-trapped interlaced elements requires nine elements, clustered in three groups, each having three elements (i.e., groups of reflectors, driven elements and directors). One aspect of the undesirable cross coupling is that the impedance of the 10 and 15 meter driven elements are adversely affected by the presence of the other closely spaced driven elements. Another aspect of the undesirable cross coupling is that the directivity pattern on 10 and 15 meters cannot be made appreciably better than the directivity pattern achievable without 10 and 15 meter director elements.
One attempt to overcome the problem of driven elements adversely affecting the impedance of one another is to employ so-called "sleeve" driven elements, in which only one of the three driven elements is directly driven and the other two driven elements, closely spaced to the directly driven element, are parasitically driven. However, overcoupling, resulting from their close proximity, results in narrowing the effective bandwidth of such driven elements.
One attempt to overcome the problem of lack of improved directivity on 10 and 15 meters is to employ more than one director for those frequency bands. However, the use of additional antenna elements adds to the wind load, weight and cost of the antenna.
Various other multiband configurations are known in the art including log periodic arrays and combination log periodic and Yagi-Uda designs in which the driven elements consist of a log periodic cell and the remaining elements include trapped and/or non-trapped interlaced parasitic elements. While providing continuous frequency coverage, log periodic arrays suffer from poor directional performance. One prior art triband beam design employs non-trapped reflector elements, log-cell driven elements and a trapped director. However, such a design requires an extra driven element (the log-cell requires four elements to cover three bands) and the trapped director cannot be optimally spaced for the multiple bands on which it operates.
Thus, an elusive goal has been to provide a triband beam or quad that has the performance of separate beams and quads each dedicated to a specific band (i.e., "monoband" beams and quads).